sticky_header [nfc]: Cut redundant assert on header size

It's already a fact that the header's size in each dimension is
non-negative and finite; the framework asserts that in the `layout`
implementation (via debugAssertDoesMeetConstraints).

So that includes `headerExtent`; and then `paintedHeaderSize` is
bounded to between zero and that value.

Author: gnprice

lib/widgets/sticky_header.dart

@@ -566,8 +566,8 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
 
     if (header != null) {
       header!.layout(constraints.asBoxConstraints(), parentUsesSize: true);
-
       final headerExtent = header!.size.onAxis(constraints.axis);
+
       final double headerOffset;
       if (_headerEndBound == null) {
         // The header's item has [StickyHeaderItem.allowOverflow] true.
@@ -578,8 +578,6 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
         final paintedHeaderSize = calculatePaintOffset(constraints, from: 0, to: headerExtent);
         final cacheExtent = calculateCacheOffset(constraints, from: 0, to: headerExtent);
 
-        assert(0 <= paintedHeaderSize && paintedHeaderSize.isFinite);
-
         geometry = SliverGeometry( // TODO review interaction with other slivers
           scrollExtent: geometry.scrollExtent,
           layoutExtent: geometry.layoutExtent,